The present invention generally relates to wireless communications receivers, and more particularly relates to techniques for processing received spread spectrum signals containing two or more simultaneously transmitted signals of interest.
In wireless communication systems that use Code-Division Multiple Access (CDMA) technology, the dispersive nature of the propagation channel often leads to a loss of orthogonality between signals that are transmitted using different channelization codes. As a result, after the signals are despread by the CDMA receiver, the code-separated signals leak into one another's symbol estimates, giving rise to multi-user interference (MUI) between the codes.
A traditional technique used to mitigate the MUI problem is to perform linear equalization, whereby the effect of the dispersive channel is partially undone. This process at least partially restores orthogonality between a signal of interest and other signals transmitted using different channelization. However, because these techniques are based on inverting the propagation channel, which generally conflicts with the goal of avoiding noise enhancement, the equalization process cannot typically fully restore orthogonality.
Another known approach for addressing inter-code MUI is to perform joint demodulation or maximum-likelihood detection (MLD) over the set of parallel codes to be received. Significant performance gains have been demonstrated using joint demodulation. The joint demodulation metric actually exploits the code leakage products, thus they no longer constitute a performance impediment.
Symbol-level joint demodulation is considered in U.S. Pat. No. 7,668,227, issued 23 Feb. 2011 (“the '227 patent”), and in U.S. Patent Application Publication No. 2011/0075767, published 31 Mar. 2011 (“the '767 Publication). The entire contents of each of the '227 patent and the '767 Publication are incorporated herein by reference, as each describes apparatus and techniques suitable for modification according to the techniques described herein.
These references describe approaches in which a received signal is first despread to obtain symbol estimates at different time lags. The despread values at the different delays are then linearly combined to suppress interference terms not included in joint demodulation. Finally, joint demodulation of the parallel codes of interest is performed. Code-specific equalizer weights are used to account for the post-despreading code leakage structure. In the '227 patent, all of the codes of interest are jointly detected at once, resulting in a single-stage structure referred to as code-specific joint detection generalized Rake (CS-joint demodulation-GRake). In the '767 Publication, a multi-stage structure is employed in which joint demodulation is performed on smaller groups of codes in each stage and a restricted number of candidates is maintained from stage-to-stage in order to reduce complexity.
In both the '227 patent and the '767 Publication, code-specific equalizer weights are used to account for the post-despreading code leakage structure. The code-specific processing accounts for the time-variant spreading waveform, which arises from the fact that the long scrambling code is much longer than a single information symbol and thus changes from one symbol period to the next. Code-specific processing is also considered in U.S. Patent Application Publication 2005/0111528, published 26 May 2005 (“the '528 Publication), the entire contents of which are also incorporated herein by reference. In the system described in the '528 Publication, however, only one symbol is detected at a time when linear equalization is used.
An alternative joint-demodulation approach to CDMA is described in S. Verdú, Multistage Detection, Cambridge University Press, Cambridge, UK, 1998 (hereinafter “Verdú). With this approach, the received chip sample sequence for each code is matched to the code-specific effective channel and despread. The per-code symbol estimates are then jointly demodulated.
One shortcoming of several of the approaches to joint demodulation summarized above is inferior performance in the presence of interference from overhead channels. The symbol-level approaches mentioned above account for dispersion, but do not adequately account for interference from overhead. The chip-level approach described by Verdú partially accounts for dispersion but does not account for interference from overhead. Especially in the context of the demodulation of high-speed data channels, “overhead” channels are generally understood to include pilots, control channels, and/or dedicated channels, e.g., voice. In the discussion that follows, however, in the context of a particular joint demodulation process the term “overhead” is meant to refer to any channels or signals other than the signals that are jointly demodulated in the process, and thus can also include data signals that are intended for the receiver of interest but that are not subjects of the particular joint demodulation process. In the presence of such overhead, insufficiently mitigated code leakage from the overhead codes to the subject signals of a demodulation process can severely hamper demodulation performance. In extreme situations, especially where a large proportion of the total signal power is allocated to overhead channels, the achievable throughput of a wireless system may be severely limited. Accordingly, improved techniques for processing received spread spectrum signals containing two or more signals of interest and one or more interfering signals are needed.